In hospitalised patents Staphylococcus aureus is an important cause of infections associated with indwelling medical devices such as catheters and prostheses (Maki, 1982; Kristinsson, 1989) and non-device related infections of surgical wounds. A recent significant increase in isolates from European and U.S. hospitals which are resistant to several antibiotics and the potential threat of emergence of vancomycin resistance in S. aureus has reinforced the importance of developing alternative prophylactic or vaccine strategies to decrease the risk of nosocomial infections due to S. aureus.
Initial localised infections can lead to more serious invasive infections such as septicaemia and endocarditis. In infections associated with medical devices, plastic and metal surfaces become coated with host plasma and matrix proteins such as fibrinogen and fibronectin shortly after implantation (Baier, 1977; Kochwa et al, 1977; Cottonaro et al, 1981). The ability of S. aureus to adhere to these proteins is believed to be a crucial determinant for initiating infection (Vaudaux et al, 1989, 1993). Vascular grafts, intravenous catheters, artificial heart valves and cardiac assist devices are thrombogenic and are prone to bacterial colonization. S. aureus is the most damaging pathogen of such infections.
Fibrin is the major component of blood clots and fibrinogen/fibrin is one of the major plasma proteins deposited on implanted biomaterial. There is considerable evidence that bacterial adherence to fibrinogen/fibrin is of importance in initiation of device related infection. (i) S. aureus adheres to plastic coverslips coated in vitro with fibrinogen in a dose-dependent manner (Vaudaux et al, 1989) and to catheters coated in vitro with fibrinogen (Cheung and Fischetti, 1990). (ii) The organism binds avidly via a fibrinogen bridge to platelets adhering to surfaces in a model that mimics a blood clot or damage to a heart valve (Herrmann et al., 1993). (iii) S. aureus can adhere to cultured endothelial cells via fibrinogen deposited from plasma acting as a bridge (Cheung et al., 1991). This suggests that fibrinogen could have a direct role in promoting invasive endocarditis. (iv) Mutants defective in a global regulatory gene sar have reduced adherence to fibrinogen and have reduced infectivity in a rat endocarditis infection model (Cheung et al., 1994). While this is indicative of a role for adherence to fibrinogen in initiating endocarditis it is by no means conclusive because sar mutants are pleiotropic and could also lack other relevant factors.
A receptor for fibrinogen often called the "clumping factor" is located on the surface of S. aureus cells (Hawiger et al., 1978, 1982). The interaction between bacteria and fibrinogen in solution results in instantaneous clumping of bacterial cells. The binding site for clumping factor of fibrinogen is located in the C-terminus of the gamma chain of the dimeric glycoprotein. The affinity for the fibrinogen receptor is very high (Kd 9.6.times.10.sup.-9 M) and clumping occurs in low concentrations of fibrinogen. It is assumed that clumping factor also promotes bacterial adhesion to solid-phase fibrinogen and to fibrin.
Clumping factor has eluded previous attempts at molecular characterisation. Reports of attempts to purify clumping factor described molecules with molecular masses ranging from 14.3 kDa to 420 kDa (Duthie, 1954; Switalski, 1976; Davison and Sanford, 1982; Espersen et al., 1985; Usui, 1986; Chhatwal et al., 1987; Lantz et al., 1990) but none were followed up with more detailed analysis. Fibrinogen is often heavily contaminated with IgG and fibronectin and unless specific steps were taken to eliminate them these studies must be suspect.
More recently it has been shown that S. aureus releases several proteins that can bind to fibrinogen (Boden and Flock, 1989, 1992, 1994; Homonylo McGavin et al., 1993). One of these is probably the same as the broad spectrum ligand binding protein identified by Homonylo McGavin et al., (1993). Another is coagulase (Boden and Flock, 1989), a predominately extracellular protein that activates the plasma clotting activity of prothrombin. Coagulase binds prothrombin at its N-terminus and also interacts with fibrinogen at its C-terminus (McDevitt et al., 1992). However, a hypothesis that the cell-bound form of coagulase is the clumping factor was disproved when coagulase-defective mutants were shown to retain clumping factor activity (McDevitt et al., 1992). There is no evidence that the fibrinogen binding region of any of these proteins is exposed on the bacterial cell surface and consequently there is no evidence that any is clumping factor.